An error only led to a death in 5.2 per cent of these cases, which was the equivalent to nearly 12,000 preventable deaths in hospitals in England every year.

It found that errors of omission were more frequent than active mistakes.

Dr Helen Hogan, who led the study, said: “We found medical staff were not doing the basics well enough – monitoring blood pressure and kidney function, for example.

“They were also not assessing patients holistically early enough in their admission so they didn’t miss any underlying condition. And they were not checking side effects ... before prescribing drugs.”

“Hospitals must learn from careful analysis of preventable deaths and make every effort to avoid [them]."

She added: “The NHS in the future is going to have to look after very frail elderly patients as their numbers increase. Our systems are not robust enough to ensure we avoid harming them.”

The study, based on analysis of 1,000 deaths at 10 NHS trusts during 2009, found that most of the patients who died were elderly and frail and suffering from multiple conditions although some were aged in their 30s and 40s.

It suggested that more supervision by senior consultants was required to ensure junior doctors carried out proper assessments.